When I used to clone my Windows Thinkpad HD, I used a utility (DOS only) which "mirrored" my original drive by copying/mapping exactly, sector by sector, so the cloned drive was for all intents and purposes, exactly the same to my laptop, other than the extra space. At least that's what I think of when I hear the term "mirroring a HD." Copying installed applications and the OS itself just don't work in the ugly world of Windows.
Which brings me to a question about cloning the MacOS... I used to just copy the system files, and any application folders in the good old days of OS 8 and earlier, and they worked, even on a different Mac... can the HD from a current Mac just be copied to an external drive, and all the applications still work, and the computer boot from the copied system? Or does it have to be "cloned" like Windows?